This invention relates generally to apparatus and a method for handling sheet material by electric field attractive forces and more particularly, provides apparatus and a method that can be selectively arranged above or below a stack of sheets for unstacking a desired number of sheets from either the top or the bottom of the stack without disturbing the remaining sheets of the stack and delivering the unstacked sheet or sheets to a predetermined location.
Sheet feeding from a stack largely has been performed using frictional forces between a sheet of the stack and means such as rollers. The rollers are urged against the top most sheet of the stack and the frictional forces between the rollers and top sheet are greater than between the top sheet and succeeding lower sheets. This differential in frictional forces is used to remove only the top sheet when the rollers are rotated. With such means, unstacking a sheet, especially automatically, can be a problem because of differences in the weight, consistency, porosity, folds or wrinkles and other quality characteristics of the sheet material, such as paper. Care has to be undertaken to use sheet material of uniform quality or to unstack the sheets manually where they are inconsistent in quality.
These sheet feeding or unstacking devices generally operate only to remove the top sheet of the stack and are unable to unstack the bottom sheet of the stack because to do so requires counteracting the weight of the stack. In many instances, means to select, unstack, and transport such a bottom sheet would be more desirable than operating upon the top of the stack.
No apparatus is known that will unstack a desired number of sheets such as one or two sheets from a stack.
It is known to remove sheets from the top of a stack using electrostatic principles. U.S. Pat. No. 3,726,520 discloses charging a film with a charged corona and extending the charged film over the top of a stack of sheets. The high voltage (5-6 Kilovolts) electrostatic charge on the film attracts the top-most sheet of the stack to the film. Thereafter, the film is withdrawn from over the top of the stack carrying the top sheet with it. Alternatively, U.S. Pat. No. 3,726,520 discloses charging a belt with charged coronas, raising a stack of sheets to a position with the top-most sheet directly under the charged portion of the belt so that the top-most sheet is attracted to the belt portion and lowering the stack. The belt then is rotated to carry the unstacked sheet to the desired location.
It is also known to convey and hold sheets using electrostatic principles. U.S. Pat. No. 4,244,465 discloses a belt formed of a base material carrying two groups of interdigitated and equally spaced apart electrodes. The two groups of electrodes are insulated from one another and are overcovered by a protective layer. The two groups of electrodes are energized to an undisclosed high voltage through contacts extending along the underside of the belt to produce an electrostatic field over the surface of the belt. A sheet of material placed on the outer surface of the belt is retained in relative position on the belt by the attractive force of the electrostatic field.
Apparatus that may be positioned above or below a stack of sheet material for removing a desired number of sheets from the top or bottom of the stack and a method for accomplishing same is not disclosed in the art.